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Abstract
Poly(hydroxyalkanoates) (PHAs), a class of bacterial polyesters consisting of homopolymer poly(3-hydroxybutyrate) (PHB) and its copolymers with 3-hydroxyvalerate, 3-hydroxyhexanoate (HHx), and 4-hydroxybutyrate have the potential for replacing polyolefins in many applications. Furthermore, these polymers are capable of biodegrading in all environments tested. However, PHAs suffer through several limitations which hinder successful commercialization. Thermal instability, slow-nucleation rate, brittleness, and aging due to secondary crystallization all limit the applications and processibility of these polymers. In this work, novel additives were developed to improve the thermal and mechanical properties of PHAs. Specifically, three class of additives were investigated: plasticizers, nucleating agents, and impact modifiers. These additives were blended into the polyester via melt-extrusion to analyzed for improvement in mechanical properties, the mechanism for improvement, and the end-of-life fate of the resulting blends. First, Hansen solubility parameters (HSP) of bioplastics were determined through a novel calculation method to expedite additive screening. Next, HSP were leveraged to develop novel biobased plasticizers derived from furan-2,5-dicarboxylic acid (FDCA). Esters of FDCA were synthesized in high yields, blended into the polymer, and evaluated as plasticizing agents compared to a phthalate plasticizer. Results demonstrated that furan-2,5-dicarobxylates were able to match or exceed the phthalate plasticizer in: Tg depression, mechanical performance, leaching behavior, and phase separation studies. Next, indigo dye was discovered as a melt-miscible nucleating agent capable of improving the crystallization rate and nucleation density of PHB-co-HHx. Results show that indigo blends were able to nucleate the polymer to the same efficiency as orotic acid, the best known nucleator for PHB-co-HHx, despite being added at 1/10th of the concentration. At elevated temperatures, indigo melted into the polymer and self-assembled into small particles upon cooling, which initiate polymer crystallization. Lastly, improvement in impact properties of PHB and PHB-co-HHx was targeted by blending Terratek FX1515 and Terratek GDH-B1FA into these polymers. Addition of impact modifier resins improved the impact properties of the polymers by 5.5x and 23x respectively, which were higher than any values previously reported in literature. Finally, the end-of-life fate of these materials evaluated through respirometry revealed that the prepared blends were capable of biodegrading under industrial composting conditions.